As methods for packaging semiconductor chips directly on a packaging substrate by face-down bonding, commonly used are flip-chip bonding in which bumps are formed at electrode areas of a semiconductor chip followed by soldering to a packaging substrate, and a connecting method in which bump electrodes provided on a semiconductor chip are coated with a conductive adhesive and are bonded to electrodes of a packaging substrate.
An anisotropic conductive adhesive having conductive particles dispersed therein is also available as a means by which electronic component parts such as semiconductor chips are electrically interconnected to a packaging substrate by mechanical electrode connection. More specifically, an adhesive film is provided between electronic component parts and electrodes or circuits and then a pressing or heat-pressing step is taken whereby the electrodes of the both are electrically interconnected to one another and at the same time an insulation performance between adjacent electrodes is imparted, thus the electronic component part devices are bonded and fastened to the circuits. This method of packaging by mechanical electrode connection is presently applied to glass substrates, and also studies are being made on its application to wiring boards made of glass-cloth reinforced resins having high general-purpose properties.
In addition, as a method of connecting electronic component parts such as semiconductor chips electrically to a packaging substrate by mechanical electrode connection, a method is also proposed in which gold bumps are formed on electrodes of a semiconductor chip, then mechanically brought into contact with gold electrodes provided on the side of a packaging substrate and at the same time held and fastened by a thermosetting or photosetting adhesive.
The wiring boards made of glass-cloth reinforced resins, used as packaging substrates, can enjoy a higher wiring density and also are economically feasible for multi-layer wiring, thus they are most commonly put into used as wiring materials. However, conventional wiring boards made of glass-cloth reinforced resin of FR-4-grade have a larger coefficient of linear expansion in the X-and Y-directions than chips and also the wiring boards made of glass-cloth reinforced resins have a high modulus of elasticity. Hence, the heat stress at the time of heat cycling, produced in packaging structure of packaging substrates, is so large as to cause a lowering of connection reliability of the packaging structure. There has been such a problem.
There has been another problem that, since a glass cloth is used as a reinforcing material, the surface of electrodes formed on the surface of the wiring board undulates periodically and hence the electrodes have uneven height to have a low connection reliability.